The use of earplugs and earmuffs are the two most useful ways to protect against hearing loss in those environments where noise levels are not able to be controlled within safe limits. In those areas where the use of earplugs is either impossible or impractical, the use of earmuffs provides a means of reducing sound intensity, in most instances to a degree even greater than that provided by the use of earplugs. Other uses for noise excluding hearing protectors include producing quiet for study, sleep, or audio purposes.
Earmuffs have advantages for intermittent use where continuous insertion and removal of earplugs would be annoying or impractical. Also, earmuffs tend to deliver higher in-field noise protection in many high frequency noise environments than most earplugs. Additional preference for earmuffs include use outdoors in cool weather and use in dry climates.
Generally, earmuffs have poorer low frequency attenuation values than earplugs. Part of the problem is because at lower frequencies of 125 to 1000 Hz the earmuff vibrates upon the earmuff cushion and flesh in a pumping mode. Most cushions are selected of a soft combination of materials to achieve conformation to the head about the ear and claim comfort because of this ease of conformation.
Most earmuffs are made up of a band section, a cup section, and a cushion section. The band section extends between the pair of muffs, and holds the muffs snugly against the head of the wearer. The cup section is typically filled with foam material, and in this combination of cup and foam is where the sound attenuation takes place. The cushion section extends around the edge of the cup, and this cushion serves two purposes, to provide comfort to the wearer, and to form a seal to assist in keeping unwanted noise away from the wearer's ears.
There is a constant search for ways to improve the comfort, sound attenuation characteristics, appearance and designs of these earmuffs (note, for example, U.S. Pat. Nos. 2,801,423; 4,260,575; 4,465,159; 4,471,496; and 4,682,374). In one of these patents, Shaw et al. U.S. Pat. No. 2,801,423, the cushion comprises a covering of pliable or flexible but non-elastic material which forms a chamber around the periphery of the rigid cup. This chamber is substantially gas evacuated and partially filled with a liquid.
Shaw et al. later redefines the preferred wall material as being polyvinyl chloride having a wall thickness of about 0.005 to about 0.01 inch and/or a dynamic Young's modules of about 5.times.10.sup.3 p.s.i. FIG. 1 shows the typical attenuation achieved by an adaptation of this patent. The figure shows ANSI S3.19 Real ear attenuation vs. Calculated Attenuation (C) for Safety Supply Model 258 Ear Muffs (Liquid Cushions as Per U.S. Pat. No. 2,801,423).
The broken line on the graph indicates the calculated values and the solid line the real ear values. Depths of 0 are formulated by the following formula: ##EQU1##
The present invention is directed to not only products, but materials and methods for producing such earmuffs which addresses the above concerns.